Named after the creamy sperm-like fluid found in its head, sperm whales are one of the few animals that can hunt and kill adult colossal squid. It may be bad news for the Antarctic’s krakens, but it is good news for us&colon; the whales’ deep-sea depredations function as a carbon sink, slightly easing the effects of anthropogenic climate change.

Poop power

Almost all marine life is found within 200 metres of the surface, the so-called photic zone. In this sunny region there is enough light for microscopic plants called phytoplankton to photosynthesise, absorbing carbon dioxide. In turn, the phytoplankton support a network of animals that feed on them and each other.

Earlier this year it was shown that Southern Ocean baleen whales help keep this process going by releasing huge amounts of iron in their faeces.The Southern Ocean is short of iron, limiting the amount of life it can sustain, but these injections of iron help out.

Now Trish Lavery of Flinders University in Adelaide, South Australia, and her colleagues have gone a step further. They have found that while the baleen whales merely help keep the iron cycle going, sperm whales actually inject iron into it by hunting their prey at great depths and then defecating when they return to the photic zone. In effect they ferry iron from the depths of the sea to the surface, where the phytoplankton can use it.

Based on existing studies of sperm whale behaviour and anatomy, Lavery and colleagues calculated that the 12,000 sperm whales living in the Southern Ocean collectively eat 2 million tonnes of prey each year – including 60 tonnes of iron.

Of this, about 36 tonnes ends up in the photic zone. That keeps swathes of phytoplankton going there, taking in CO2 from the atmosphere, that otherwise couldn’t make a living.

Of all the carbon taken in by the phytoplankton, between 20 and 40 per cent ultimately sinks to the bottom of the ocean as various forms of waste. Lavery’s team calculated that 400,000 tonnes of carbon gets dumped in this way every year as a result of the sperm whales’ activities – far more than the estimated 160,000 tonnes the whales release by breathing.

Deep hunter

All of which raises the question, how do sperm whales bring down such monstrous prey? No one has ever seen a sperm whale attack a colossal squid, but we can still work some of it out.

The whales have one advantage&colon; in the cold Antarctic, colossal squid are rather slow-moving, lying in wait and ambushing their prey rather than actively hunting them. A hungry sperm whale might not have to chase very hard.

The spermaceti also cushions the whales’ heads, particularly in males, which have much more of the stuff than females do. This allows fighting males to ram each other (or, in Moby-Dick, ships). However, there is no evidence of them ramming prey.